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3 Copyright 2013 Resonate, Inc. All rights reserved. Resonate Incorporated and its licensors retain all ownership rights to the Central Dispatch, CDConsole, CDvLB, and Global Dispatch computer programs (hereinafter collectively called Resonate Software) and their documentation. Use of Resonate Software is governed by the license agreement accompanying your original media. Resonate is a registered trademark of Resonate, Inc. The Resonate logo, Resonate Global Dispatch, Resonate CDConsole, the Resonate CDConsole logo, Resonate Central Dispatch, Resonate CDvLB, and Resource-based Scheduling are trademarks of Resonate, Inc. in the United States and/or other countries. All other brand or product names used in this document are trademarks or registered trademarks of their respective companies or organizations. Resource-based Scheduling technology is protected under U.S. Patent 5,774,660. THE INSTRUCTION AND INFORMATION REGARDING THE PRODUCTS IN THIS GUIDIE ARE SUBJECT TO CHANGE WITHOUT NOTICE. COMPANIES, NAMES, AND DATA USED IN ANY EXAMPLES HEREIN ARE FICTITIOUS UNLESS OTHERWISE NOTED. ALL STATEMENTS, INFORMATION, DECLARATIONS AND RECOMMENDATIONS IN THIS MANUAL ARE BELIEVED TO BE ACCURATE BUT ARE PRESENTED WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED. USERS MUST TAKE FULL RESPONSIBILITY FOR THEIR APPLICATION OF ANY PRODUCTS.

6 Preface Purpose Preface Resonate Global Dispatch is a WAN-based, multi-site, service level control solution that provides high availability and optimum performance for geographically distributed business applications. It enables multiple, dispersed points of presence (POPs) to act as a single, highly available system and directs user requests to the POP best able to maximize service levels. Additionally, it can enhance a user s experience by routing them to the POP with the content most relevant to their needs or it can direct intranet users to the POP closest to their location to save on WAN costs. This section contains the following topics: Purpose Purpose Availability The purpose of this application note is to provide detailed information about Intelligent Multi Site Service Level Control. Availability A soft copy of this application note is available on the Resonate support web site at: 3 White Paper

7 Abstract Availability Abstract As mission-critical business activities become more dependent on geographically dispersed information, services, and computing resources, internal and external networks are the lifelines of enterprises. Across the global corporate network or over the Internet, employees, partners and customers expect fast, reliable service from IP-based applications. This distributed environment becomes more indispensable to business operations, its expanding complexities from applications to geographies, networking infrastructures, server systems, and user bases making its deployment and management increasingly difficult. System/network administrators and business managers struggle to match existing system resources with client requirements while trying to prepare for imminent and exponential increases in demand for global applications and services. White Paper 4

8 Overview Site Availability Overview By gathering and using latency, load, and availability information, Global Dispatch software directs client requests to the physical site best suited to respond. As illustrated in Figure 1: Global Dispatch Configuration, Global Dispatch combines multiple physical sites into a single (or more, if needed) virtual hostname, such as or company.com. Clients use the virtual hostname to access resources that may be available across multiple physical sites. When the local DNS server of a client makes an address resolution request for a virtual hostname, Global Dispatch responds with the IP address of the most appropriate physical site based on the latency and/or load information it receives from Global Dispatch agents installed at each site. Global Dispatch uses the following metrics and policies to determine the site best suited to respond to client requests: Site Availability Response Time Site Load Persistence Directed Traffic Tables Site Availability Global Dispatch actively monitors the availability of each site through Global Dispatch agents and feedback from the Central Dispatch agents (if Central Dispatch is used as the local traffic management solution). Requests are routed around detected, failed sites and are only sent to the available resources. In the event of a downed site, clients are re-routed until the site becomes available again, at which time it is automatically re-integrated into the list of available sites. For example, if a database is unavailable at a particular site but the site is still available, Global Dispatch software can be configured to automatically direct users to alternate sites where the required resources are available. White Paper 5

9 Overview Response Time Figure 1: Global Dispatch Configuration Response Time Global Dispatch measures the network latency between the client's local DNS server and each site. Through triangulation between the client s local DNS server, the Global Dispatch Scheduler and Global Dispatch agents, the client is sent to the site with the fastest response time (that is, the smallest latency). Site Load Global Dispatch measures the load at each site and distributes incoming requests to the least-loaded sites. Integration with Central Dispatch enables more precise load statistics to be captured, such as CPU load across the site and other custom-defined criteria allowing administrators to address specific load requirements needed in their environment. 6 White Paper

10 Overview Persistence Persistence For a designated period of time from the first request, all subsequent requests from the same client can be assigned to the same site that handled the initial request. This "sticky session" accommodates requests that generate and utilize state information (typically storing it in the server's database). With persistent session support, server databases need only be synchronized periodically, avoiding the high overhead and cost associated with real-time synchronization of content. Directed Traffic Tables Global Dispatch enables designated traffic to be directly routed to a specified site, avoiding the standard tests for the best-suited site. For example, a Web site administrator may want to designate a particular physical site to handle all requests from customers of a particular ISP, a particular region of the world, or a particular company. The intelligent scheduling capabilities and an easy-to-use management console enable administrators to efficiently direct requests across multiple sites and also accommodate diverse company requirements. Administrators gain control over geographically dispersed traffic to deliver the best experience to its users, save costs by optimally allocating server resources and reduce traffic on private backbone WANs. To effectively manage WAN traffic and server resources, Global Dispatch software integrates three major components: A Scheduler that receives and resolves client requests. Agents that monitor and communicate physical POP status. A Global Dispatch Manager console that allows configuration and gathering of site statistics. These three components integrate tightly to deliver a responsive system to maximize service levels for globally distributed traffic. White Paper 7

11 Global Dispatch Scheduler Intelligently managing global traffic Global Dispatch Scheduler Intelligently managing global traffic Global Dispatch Scheduler (item C in Figure 1) ensures that traffic is directed to the most available and appropriate site. The Global Dispatch Scheduler takes the client requests for the virtual hostname through the client s local DNS server and DNS hierarchy. It then directs the client to the most appropriate physical site by first determining the optimal site and then returning the IP address (or Virtual IP address) of that site. A single IP address can be returned for the address resolution or multiple IP addresses can be returned for additional failover. In addition, the Scheduler provides the intelligence to route clients around failed sites or to direct requests based on predefined rules and policies. These predefined rules allow Global Dispatch to send clients to sites that are closest in proximity, least loaded or offering the best response time. Global Dispatch observes the following policies, in order, when directing clients to the most appropriate site. See Figure 2: Traffic Scheduling decision flow. With Directed Traffic Table If any directed traffic table entries have been put in place (For example, all traffic from a designated IP address range mapped to a specific server), then these rules are used first to arrive at a target IP address. With Persistence Table The Scheduler then checks the persistence table at this point to determine if a "sticky session" was previously created that still applies to this request. The persistence tables can be synchronized across Schedulers at administratordefined intervals to ensure session state is maintained. 8 White Paper

12 Global Dispatch Scheduler Intelligently managing global traffic Figure 2: Traffic Scheduling decision flow With Load Balancing Rules If neither persistence nor directed traffic features have been enabled, the Scheduler chooses a physical POP based on the latency and load information it receives from the Global Dispatch agents installed at each POP. The following screenshot show the load balancing rules that you can select in Global Dispatch: Figure 3: Load balancing rules White Paper 9

13 Global Dispatch Scheduler Round Robin Round Robin Basic round-robin selection of sites based on all available sites. Weighted Round-Robin Round-robin selection of sites adjusted using the site weights to send proportionally more traffic to POPs with a higher weight value. This helps if some sites have a higher capacity and ability to respond to client requests. Latency Only Global Dispatch chooses the POP with the lowest latency value unless the POP is unavailable. To maximize performance, the Global Dispatch Scheduler maintains a cache in RAM where it stores latency information for client DNS servers. When the Scheduler gets a request from a client, it first checks the cache for saved latency information about that particular DNS. The cache is automatically flushed at default or user-specified intervals. If the request results in a cache miss, then the Scheduler generates requests to the Global Dispatch agents for measuring the latencies between the agents and the requestor's local DNS. Load Only When using the load only method, Global Dispatch uses an algorithm that calculates a quota and gauges the ability of a POP to service a request. The Scheduler assigns a quota to each POP participating in a virtual host site. This quota will be dynamically modified by changes of the load so the higher the quota, the better its ability to serve traffic. The quota will be higher if the site load is low and the quota will be lower if the site load is high. Both Latency and Load Global Dispatch sorts the POPs based on their latency value in the increasing latency order. Global Dispatch will then run through this list and select a POP which has not yet filled its quota of hits. This mode will try to send users to the closest POP, as long as the load distribution is even. 10 White Paper

14 Global Dispatch Scheduler Quick Response to Requests Quick Response to Requests When latency measurements are chosen, Global Dispatch has the ability to take immediate action and send an IP address back to the local DNS server with round robin without having to wait for the latency measurements to be returned. This mode of operation is especially useful in providing a quick response to client requests while still enabling latency measurements to be factored into subsequent decisions. The Scheduler quickly returns an IP address back to the client s local DNS server with round-robin while latency measurements are performed in the background and stored in the cache for future requests. Testing Configuration before Deploying Another mode of operation for the Scheduler is Shadow Mode. The Shadow mode enables administrators to test the Global Dispatch configuration before setting it into actual production. In the Shadow mode, Global Dispatch software responds to DNS queries using a simple round-robin scheduling method. Simultaneously, it logs the results of "what-if" calculations to show which POP would have been selected had certain rules been activated. Shadow Mode helps administrators better understand site traffic and routing patterns at initial site setup. Additionally, it provides a means to safely test adjustments that may be warranted because of fluctuations in site activity. Integration with Authoritative DNS Server The Scheduler can be used in conjunction with the authoritative DNS servers either as a delegate to the authoritative server or s a proxy to the authoritative DNS server to resolve A-records (common hostname request for IP address). When Global Dispatch is a delegate of the authoritative server (See Figure 4: Global Dispatch as a delegate to the Authoritative DNS Server), hostnames such as ftp.domain.com, are delegated as sub-domains to Global Dispatch. The delegation redirects the local DNS server of a client from the Authoritative DNS server to the Global Dispatch Scheduler for subdomains. For instance, when is requested, the authoritative DNS server redirects, or delegates, to be resolved by the Global Dispatch Scheduler. The Scheduler then responds to the local DNS server of the client with the appropriate IP address. The authoritative DNS server will still handle the domain.com request, but it will pass the sub.domain.com requests on to Global Dispatch. When the Scheduler acts as a proxy to the authoritative DNS server (See Figure 5: Global Dispatch as a proxy to the authoritative DNS server), it can also handle requests for domain.com and it is treated as a peer to the authoritative DNS White Paper 11

15 Global Dispatch Scheduler Integration with Authoritative DNS Server server. It can resolve both domain.com requests and subdomain requests, such as Any requests that Global Dispatch Scheduler cannot resolve in this mode will be requested from the authoritative DNS server and returned back to the Scheduler. This will include any non-a records. Figure 4: Global Dispatch as a delegate to the Authoritative DNS Server 12 White Paper

16 Global Dispatch Scheduler Integration with Authoritative DNS Server Figure 5: Global Dispatch as a proxy to the authoritative DNS server The Scheduler is able to direct incoming requests from the local DNS server of a client to the appropriate site based on predefined policies giving administrators ultimate flexibility in directing traffic and delivering predictable service levels. The Global Dispatch Scheduler, however, does not act alone. It gathers site information from the Global Dispatch agent to aid in its decision making. White Paper 13

17 Global Dispatch Agents Gathering valuable site metrics Global Dispatch Agents Gathering valuable site metrics Connect Global Dispatch Agents gather statistics from the physical sites to provide the Schedulers with site availability, load and response times to the client DNS servers. The agents are installed at each physical POP and are the Scheduler's point of contact for information about a POP. Agents can be loaded on any local server, which allows the Global Dispatch Scheduler to work with any local traffic management solution including Resonate Central Dispatch sites, hardware-based load balanced sites, and sites hosted on a single Web server. Each agent measures both the POP load and the latency between its POP and the client DNS server making the address resolution request. The agent, independent of DNS requests, periodically communicates load information to the Scheduler; while latency information is measured and communicated when initiated by a DNS latency request. If the load measurements cannot be completed during successive tries, the Scheduler will mark the site as down and will direct traffic to other sites. In addition, a single POP can have multiple agents, each carrying a share of the triangulation work (latency measurement) for that POP. This configuration minimizes scheduling overhead in the largest Internet environments preventing agents from being a single point of failure. When measuring site load, Global Dispatch agents offer administrators tremendous flexibility in choosing the load calculation method that is most appropriate for individual site environments. Agents determine site load in one of the following ways. See Figure 6: Determining site load. Connects to the POP and measures how long it takes to establish the connection. URL-Connect Requests a specific file from the POP and measures how long it takes to begin receiving the file. Central Dispatch CPU Load or Open Connections Collects the average CPU load and open connections across a Central Dispatch cluster and reports the information back to the Scheduler. 14 White Paper

18 Global Dispatch Agents Custom Script Custom Script Custom script runs an administrator-defined script that determines the load. The ability to create a script to return load values is an extremely flexible method to collect load metrics from specialized applications and services. Administrators can develop custom scripts incorporating non-standard metrics, returning a load value ranging from 0 to 100, to influence Global Dispatch scheduling decisions. Load Server Load server connects to an administrator-defined load server process, running on a remote host server to return a load value. The load server can run a server process to measure the load at the site and return a value back to the agent to influence the Global Dispatch scheduling decision. Agents Figure 6: Determining site load Agents also provide the Scheduler with latency information by measuring the time it takes for them to connect to the client DNS server, and thereby supply the response time between the client DNS and each Global Dispatch site. The agents use a variety of methods to gather latency between the physical site and the client DNS server. Each method can be used in different environments to White Paper 15

19 Global Dispatch Agents TCP Connect deliver reliable, latency measurement results. See Figure 7: Selecting latency measurement. TCP Connect With this method, the agent sends a TCP connection request to the client s local DNS server. The roundtrip time is measured as the latency. ICMP Ping Using the Internet Control Message Protocol (ICMP) method, the agent measures the time it takes to receive a response to a single ICMP request packet sent to the client DNS. Because only a single packet is exchanged, the ICMP method is faster and provides a more reliable measure of network latency (virtually eliminating CPU overhead) Reverse DNS Lookup With this option, the IP address of the requesting client DNS server is sent to the agents. The agents will then request a hostname lookup for that IP address from the client DNS. The time taken to get a response for this query will be measured as the latency. The Global Dispatch agents provide reliable and accurate site load and client response statistics that enable Global Dispatch to make intelligent decisions about where traffic should be directed. Using multiple agents provide failover and the ability to share the load across agents at large sites. Choosing these load and latency measurements can be easily accomplished in the Global Dispatch Manager. 16 White Paper

21 Global Dispatch Manager Easy configuration and management Global Dispatch Manager Easy configuration and management Global Dispatch Manager (See Figure 8: Easy configuration with Global Dispatch Manager) enables administrators to quickly and easily set up, configure and manage multiple, geographically distributed sites. The Global Dispatch Manager is a graphical console that gives administrators a single view into their Global Dispatch sites and allows management from local or remote locations. This tool enables administrators to quickly bring up a redundant site with an easy-to-use interface. Once the Global Dispatch sites are up and running, statistics can be gathered through the Global Dispatch Manager to provide valuable usage information that helps administrators troubleshoot and fine-tune the site configuration. The Global Dispatch Manager has been created to facilitate quick configuration of redundant sites through basic and advanced configuration screens. With error checking, pull down menus and simple check boxes, companies can ensure their multi-site configuration is created quickly and easily from the start. For additional flexibility, all configuration and commands in the Global Dispatch Manager can be executed through a command line interface. Figure 8: Easy configuration with Global Dispatch Manager The Global Dispatch Manager and command line utilities also provide site statistics to enable administrators to quickly troubleshoot problems and to 18 White Paper

22 Global Dispatch Manager Easy configuration and management proactively monitor traffic patterns. The Global Dispatch statistics program queries the Scheduler for information; it counts and refreshes statistics, and flushes the Scheduler cache as requested. To provide important system management information to administrators, Global Dispatch statistics describe the health of POPs, as well as traffic and system efficiency. Global Dispatch logs the results of all DNS requests including the source IP address of the DNS query, the physical POP chosen, and the IP address returned. The Global Dispatch Manager provides easy access to the following available statistics (See Figure 9: Proactive management with Global Dispatch statistics): Current size of the cache in bytes or number of entries. Number of hits and misses on the cache. Average time taken for Global Dispatch to respond to DNS requests. Number of requests for a particular host name. Number of times a particular POP was selected in response to a DNS request. Current status of all physical POPs. Number of times a latency measurement failed from each physical POP. Global Dispatch statistics can be continuously displayed to provide on-going site evaluation. Figure 9: Proactive management with Global Dispatch statistics White Paper 19

23 Bringing it all together Easy configuration and management Bringing it all together All the Global Dispatch elements tie tightly together to provide the highest level of availability and performance while delivering the best user experience. When a client connects to a Web site, the site name (For example, or resonate.com) requires resolution to an IP address. This process is illustrated in Figure 10: Resolving a virtual hostname using latency and load. The process steps include the following: 1. The client sends a request to its local DNS. 2. If the requested site information is not cached in the local DNS, the client s local DNS server sends a request to the requested site's authoritative DNS server (or Global Dispatch Scheduler if the Scheduler is acting as a proxy) through the DNS hierarchy. The authoritative DNS server examines the subdomain name requested and passes the IP address of the Global Dispatch Scheduler back to the local DNS server. The local DNS server then communicates with the Global Dispatch Scheduler directly. 3. Global Dispatch determines if the scheduling information has been defined through the directed traffic table or persistence table. If these features are enabled, skip to The Global Dispatch Scheduler uses the load balancing policy configured (load, latency, latency and load, round-robin, or weighted round-robin) to determine the most appropriate site for the client. Load and latency metrics are captured from the Global Dispatch agents at each site and returned back to the Scheduler. 5. The IP address (or Virtual IP) of the appropriate POP is returned to the local DNS server. Multiple IP addresses are returned if this feature is enabled. 6. The local DNS server then passes this address (or addresses) back to the client. 7. Client traffic is then directed to the most available, appropriate POP. Note: Global Dispatch also supports multiple virtual host names at each POP, allowing fine granularity in directing traffic to meet varying browser access patterns or unique server configurations. 20 White Paper

24 Bringing it all together Easy configuration and management Figure 10: Resolving a virtual hostname using latency and load White Paper 21

25 Global Service Level Control Easy configuration and management Global Service Level Control A business success is dependent on the reliable access of information, and it can't afford a network outage or slow response times. Resonate Global Dispatch affords a WAN-based, multi-site, service level control solution that provides high availability and optimum performance for geographically distributed business applications. The Global Dispatch Scheduler enables intelligent management of globally distributed traffic to ensure traffic is always directed toward the most available, responsive sites while the agents gather the valuable site metrics that help in the decision making process. The Global Dispatch Manager provides a single interface to configure, monitor and proactively manage distributed sites. Resonate Global Dispatch is the only software solution that has been uniquely designed to provide high availability and scalability without compromising performance while still offering the greatest flexibility and control over geographically distributed traffic. It s flexible, native software solution layers onto existing infrastructure, including existing DNS and local server load balancing solutions, enabling companies to quickly and easily deploy a redundant site and ultimately deliver a great end user experience. Integrating with all the Resonate products including Central Dispatch and CDConsole, Resonate Global Dispatch ensures that service levels can be easily controlled and maintained across multiple applications, services and geographies. 22 White Paper

26 Technical support Technical support If you have any trouble with Resonate Global Dispatch configuration for load balancing, you should: 1. Retry the action. Carefully follow the instructions written in the guide. 2. Check the documentation that came with your hardware for maintenance or hardware-related problems. 3. Note the sequence of events that led to the problem and the exact messages displayed. Have the documentation available. 4. If you continue to have a problem, contact Resonate Technical Support by sending an message to the following address: White Paper 23

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